Corrosion of metal and mineral scale formation are common problems in a variety of industrial settings, especially in oilfield and water treatment systems. In corrosion, a chemical or electrochemical reaction between a material, usually a metal, and its environment produces a deterioration of the material and its properties. This corrosive attack can be uniform or localized over the metal surface but generally results in undesirably shortening the useful life or utility of the metal surface.
An example of chemical attack is the air oxidation of hot steel which forms an iron oxide coating. In order to have electrochemical corrosion, it is necessary to have an (1) anode; (2) cathode; (3) electrolyte and (4) external connection.
The presence of water is essential to low temperature corrosion processes. However, pure water containing no dissolved substances is only very mildly corrosive to iron. Water containing impurities or dissolved substances can be corrosive or noncorrosive, depending on the nature of the dissolved substances. Chromates and phosphates are dissolved in water to inhibit or reduce corrosion. Other substances such as salts, acids, hydrogen sulfide, carbon dioxide, and oxygen can increase the corrosivity of the water. Generally, the water encountered in oilfield operations, in particular, contains one or more of these substances which increase its corrosivity.
Dissolved carbon dioxide further influences the solubility of magnesium and calcium carbonates. These salts sometimes precipitate on the surface of a metal pipe and form a protective coating. However, water containing "aggressive" carbon dioxide (i.e., excess carbon dioxide dissolved in water) will not deposit this protective coating. Salts dissolved in the water may act as buffers, thereby preventing the pH from reaching a low enough value to produce serious corrosion.
In addition to the impurities which are commonly found in water, temperature and velocity also influence the corrosivity of water. Seldom is a corrosion problem encountered where only one of these contributing factors is present. Consequently, the problem is complex because of these various influences and the manner in which they may interact with each other. Thus, the art continues to need new and improved methods of inhibiting metal corrosion in various aqueous environments utilizing environmentally acceptable chemistries.
In certain industries, economics often determine what metal materials of construction are selected for equipment associated with that industry. The North Sea and Alaskan oil and gas production fields are typical commercial examples. For example, mild steel is generally the metal of choice for equipment and long pipelines. Oil field waters, such as brine and formation water, present in mild steel pipes provide a corrosive environment which can cause electrochemical corrosion to occur at the solid-liquid interface. In this corrosive environment, carbon dioxide is dissolved in a brackish to brine aqueous solution with associated hydrocarbons from the production of the oil or gas but it will generally not contain dissolved oxygen. Consequently, chemical corrosion seldom occurs but electrochemical corrosion occurs at solid-liquid interfaces in nearly every instance where oilfield water contact steel equipment.
The need for a specialized corrosion inhibition is known to persons in the field of controlling the internal corrosion of mild steel surfaces associated with oil and gas production and their transportation. Protecting metal surfaces against corrosive deterioration is currently achieved through the use of multi-component corrosion inhibitor systems, which are nitrogen and aromatic compounds, such as amine and organic sulfide containing compositions. These combination corrosion inhibitors, therefore, raise environmental concerns due to their persistence or hazardous nature and biota impact on the surrounding environment and public health.
Heavy metals, chromates, phosphates, silicates and persistent film-forming materials are typical inhibitors for minimizing corrosion of iron and steel in aqueous solutions. These inhibitors all have a negative environmental impact, such as, toxicity, eutrophication and environmental persistence. Moreover the removal of these materials from the environment requires complicated and expensive processes.
There is a desire and need, therefore, for environmentally friendly (biodegradable) chemistries which provide equal or better carbon dioxide corrosion inhibition in otherwise corrosive aqueous saline environments than presently available inhibitors.
The search for environmentally acceptable carbon dioxide corrosion inhibitors for metal surfaces in contact with aqueous saline environments is well known to those skilled in the art of aqueous corrosion inhibition.
Polyaspartic acid and its salts have previously been shown to inhibit scale formation and possess dispersancy properties for calcium carbonate and phosphate in U.S. Pat. No. 5,152,902 to Koskan et al., and of calcium sulfate and barium sulfate in U.S. Pat. No. 5,116,513 to Koskan et al. These characteristics make polyaspartic acid and its salts desirably compatible with the deposit control chemistries utilized in the oil and gas production industries.
Amino acids, and notably aspartic acid, have generally been found to have little tendency toward effective corrosion inhibition for commercial use. Moreover, aspartic acid is known to be inherently corrosive at slightly alkaline pH conditions, reportedly actually accelerating corrosion at a pH of about 8. Therefore, amino acids, such as aspartic acid, although possessing desirable non-toxic biodegradable properties, generally have been avoided as corrosion inhibitors.
Researchers have reported that thermally produced polyaspartate, a synthetic polypeptide consisting of approximately 20 aspartic acid residues (apparent molecular weight of about 2000 to about 5000) was a mild inhibitor of the corrosion of mild steel coupons exposed to synthetic seawater at pH 8 under static use conditions. However, the maximum inhibition achieved reportedly was less than 30%. See, Little, et al., "Corrosion Inhibition by Polyaspartate," Surface Reactive Peptides and Polymers: Discovery and Commercialization, Sikes and Wheeler (Eds), ACS Symposium Series No. 444(1990); and Mueller et al., "Polypeptide Inhibitors of Steel Corrosion in Sea Water," Paper 274 presented at the NACE Annual Conference and Corrosion Show (1991).
U.S. Pat. No. 4,971,724 to Kalota et al., teaches that aspartic acid and polyaspartic acid demonstrate corrosion inhibiting properties on mild steel coupons in aerated, carbon dioxide-free deionized water under static use conditions providing they are fully ionized at above pH 8.9. However, pitting corrosion remained a concern until above pH 10.
Surprisingly, polyaspartic acid has now been found useful as a carbon dioxide corrosion inhibitor of ferrous metals in an aqueous saline environment that is substantially free of dissolved oxygen.